Curable flux composition and method of soldering

ABSTRACT

A curable flux composition is provided, comprising, as initial components: a resin component having at least two oxirane groups per molecule; a carboxylic acid; a fluxing agent represented by formula I: 
                         
wherein R 1 , R 2 , R 3  and R 4  are independently selected from a hydrogen, a substituted C 1-80  alkyl group, an unsubstituted C 1-80  alkyl group, a substituted C 7-80  arylalkyl group and an unsubstituted C 7-80  arylalkyl group; and wherein zero to three of R 1 , R 2 , R 3  and R 4  is(are) a hydrogen; and, optionally, a curing agent. Also provided is a method of soldering an electrical contact using the curable flux composition.

The present invention relates to a curable flux composition comprising,as initial components: a resin component having at least two oxiranegroups per molecule; a carboxylic acid; a fluxing agent represented byformula I, wherein R¹, R², R³ and R⁴ are independently selected from ahydrogen, a substituted C₁₋₈₀ alkyl group, an unsubstituted C₁₋₈₀ alkylgroup, a substituted C₇₋₈₀ arylalkyl group and an unsubstituted C₇₋₈₀arylalkyl group; and, wherein zero to three of R¹, R², R³ and R⁴ is(are)a hydrogen; and, optionally, a curing agent. The present inventionfurther relates to a method of soldering an electrical contact using thecurable flux composition.

Flux is an important tool used in the manufacture of electrical devices,including mounting of an electronic component (e.g., a semiconductorchip) onto a substrate (e.g., a printed circuit board, printed circuitcard, organic substrate, silicon interposer, another semiconductorchip).

The flip chip method is an increasingly important method used formounting electronic components onto a substrate. In one example of aflip chip method used to mount a semiconductor chip on a substrate,solder is provided (e.g., as solder balls) on contacts located on thesemiconductor chip (e.g., contact pads, contact pins). Alternatively,solder is provided on corresponding contacts located on the substrate(e.g., contact pads, copper plated through holes). A flux is applied tothe solder to remove oxide layers that may be present on the surface ofthe solder or on the surface of the contacts on the semiconductor chipor the substrate. The flux also functions to provide increased wettingof the contacts by the solder during reflow. The solder or contacts onthe semiconductor chip are then brought into physical contact with thecorresponding contacts or solder on the substrate. The solder on thesemiconductor chip and/or the substrate is then heated to reflow. Uponcooling, inter connections between the semiconductor chip and thesubstrate are formed. Typically, these inter connections aresubsequently encapsulated (e.g., with an epoxy resin) to enhancereliability of the semiconductor chip/substrate assembly. That is, theencapsulating resin helps to relieve strains that may otherwise developgiven differences in the coefficient of thermal expansion of thesemiconductor chip and the substrate.

The actual choice of the flux used in the above described process isvery important. The use of many conventional fluxes, result in theformation of undesirable ionic residues that may reduce the reliabilityof the product device. Accordingly, such undesirable ionic residues mustbe cleaned from the device. Cleaning of such devices, however, ishindered by the fact that the distance between a semiconductor chip anda substrate (e.g., printed circuit board) following formation ofsoldered inter connections is very small. This significantly complicatesthe process of removing any undesirable ionic residues formed during thesoldering process.

Conventionally, curable organic materials (typically containing organicor inorganic fillers) are used to fill the gap between the semiconductorchip and the substrate and to reinforce the solder joints electricallyinterconnecting the semiconductor chip and the substrate. Theseunderfill materials rely on capillary action to fill the gap.

The gap between the semiconductor chip and the substrate must becompletely filled to provide maximum reliability to the productelectrical component. However, when the curable organic material isapplied at the periphery of the gap between the semiconductor chip andthe substrate, voids at the center area of the gap can remain. As thesize of the electrical component shrinks (i.e., the height of the gapgets smaller), the limiting effect of capillary action causes thenon-filled center region to expand.

To address this issue, some have provided a hole in the substratecorrespondingly near the center of the gap region. Underfill material isthen supplied to the gap through this hole and to the periphery. Thisapproach, however, requires device designs to provide a region free ofcircuitry to facilitate the location of the center hole.

Another approach to the underfill issue has come to be referred to as a“no flow” process, wherein an underfill material is preapplied onto thesemiconductor chip and/or the substrate before soldering. This materialsubsequently occupies the gap between the semiconductor chip and thesubstrate upon soldering to form an interconnected component and iscured (typically by application of heat).

In such no flow processes, it is known to use an underfill material thatprovides both fluxing and encapsulating functionalities. Such materialsreduce the number of steps required to package semiconductor chips onsubstrates. That is, these materials combine the fluxing and underfillsteps into one and eliminate the need for a cleaning step.

One such no flow underfill material is disclosed by Pennisi et al. inU.S. Pat. No. 5,128,746. Pennisi et al. disclose a thermally curableadhesive having a fluxing agent for use in reflow soldering anelectrical component and a substrate, comprising an adhesive thatremoves oxide coatings from the electrical component or the substrateand at least partially cures when heated to soldering temperatures, saidadhesive consisting essentially of a thermoset resin, a fluxing agent inan amount sufficient to remove said oxide coatings from said componentor said substrate, and a curing agent that reacts with and cures thethermoset resin when the thermally curable adhesive is heated.

Another no flow underfill approach is disclosed by Chen et al. in U.S.Pat. No. 7,303,944. Chen et al. disclose a method comprising: applyingan underfill material containing an anhydride adduct of a rosin compoundas a fluxing agent over a contact pad over a surface of a substrate;placing a microelectronic device having an active surface, and aplurality of solder bumps disposed on a plurality of contact pads on theactive surface, relative to the substrate, with the plurality of solderbumps disposed within the no flow underfill material; removing a metaloxide from the solder bumps with the fluxing agent; reflowing the solderbumps by heating to a temperature that is greater than a melting pointtemperature of the solder; and curing the underfill material.

Many of the conventional no flow underfill materials like those notedare built on epoxy chemistry and rely on carboxylic acids or anhydridesto provide fluxing. Also, organic alcohols are periodically used asaccelerators, given that they react with anhydrides to form carboxylicacid fluxing agents. Carboxylic acids, however, tend to be volatileduring the soldering and encapsulating process. This is undesirable inthat it can create voids in the gap between the semiconductor chip andthe substrate that reduce the reliability of the product device.Accordingly, there remains a need for no flow underfill materials thatfacilitate the manufacture of reliable soldered and encapsulated interconnects in electrical components (e.g., inter connects between asemiconductor chip and a printed circuit board).

The present invention provides a curable flux composition, comprising,as initial components: a resin component having at least two oxiranegroups per molecule; a carboxylic acid; a fluxing agent represented byformula I:

wherein R¹, R², R³ and R⁴ are independently selected from a hydrogen, asubstituted C₁₋₈₀ alkyl group, an unsubstituted C₁₋₈₀ alkyl group, asubstituted C₇₋₈₀ arylalkyl group and an unsubstituted C₇₋₈₀ arylalkylgroup; and wherein zero to three of R¹, R², R³ and R⁴ is(are) ahydrogen; and, optionally, a curing agent.

The present invention provides a process of forming an encapsulatedmetallurgical joint, comprising: providing a curable flux composition ofthe present invention; providing a plurality of first electricalcontacts; providing a plurality of corresponding second electricalcontacts; providing a solder; applying the curable flux composition toat least one of the plurality of first electrical contacts and theplurality of corresponding second electrical contacts; placing theplurality of first electrical contacts in proximity to the plurality ofcorresponding second electrical contacts; heating the solder above itsreflow temperature forming a molten solder and exposing the moltensolder to the plurality of first electrical contacts and the pluralityof corresponding second electrical contacts; displacing the curable fluxcomposition from the plurality of first electrical contacts and theplurality of corresponding second electrical contacts with the moltensolder and forming a plurality of electrical inter connects between theplurality of first electrical contacts and the plurality ofcorresponding second electrical contacts; and, curing the curablethermosetting resin composition, encapsulating the plurality ofelectrical inter connects.

DETAILED DESCRIPTION

The curable flux composition of the present invention is designed tofacilitate the manufacture of electrical components having soldered andencapsulated electrical inter connects. For example, the curable fluxcomposition of the present invention is preferably designed to functionas a no flow underfill formulation in the manufacture of semiconductordevices.

The term “no flow underfill composition” as used herein and in theappended claims refers to a curable flux composition that exhibits bothsolder fluxing activity and latent curing to facilitate encapsulation ofsoldered inter connects.

The term “storage stability” as used herein and in the appended claimsin reference to a curable flux composition of the present invention (ina one pack system) means that the viscosity of the curable fluxcomposition increases less than 5% following storage at 55° C. for oneweek, wherein the viscosity is measured using a Brookfield DV-I+Viscometer at 20° C. using a Brookfield #S00 spindle set at 100 rpm.

The term “storage stable” as used herein and in the appended claims inreference to a curable flux composition of the present invention meansthat the curable flux composition exhibits storage stability.

The curable flux composition of the present invention comprises(consists essentially of), as initial components: a resin componenthaving at least two oxirane groups per molecule; a carboxylic acid; afluxing agent represented by formula I; and, optionally, a curing agent.Preferably, the curable flux composition contains less than 10 wt %material that volatilizes upon heating to 250° C. as determined bythermogravimetric analysis (TGA) using a temperature ramp of 10° C./min.starting at 25° C. Evolution of gases from underfill compositions tendsto cause voids in the gap between the semiconductor wafer and thesubstrate, resulting in potential reliability concerns for the productpackaged device.

The resin component used in the curable flux composition of the presentinvention includes materials having at least two oxirane groups permolecule. Preferably, the resin component used is an epoxy resin havingat least two epoxide groups per molecule, for example, substituted orunsubstituted aliphatic, cycloaliphatic, aromatic and heterocyclicpolyepoxides. More preferably the resin component is an epoxy resinselected from bisphenol type epoxy resin (e.g., bisphenol A type epoxyresin, bisphenol F type epoxy resin, bisphenol S type epoxy resin);aromatic diglycidyl ethers; aromatic multifunctional glycidyl ethers;aliphatic diglycidyl ethers and aliphatic multifunctional glycidylethers. Still more preferably the resin component is a bisphenol typeepoxy resin. Most preferably the resin component is a bisphenol A typeepoxy resin. The curable flux composition preferably comprises 10 to 99wt % (more preferably 20 to 90 wt %; still more preferably 30 to 75 wt%, most preferably 30 to 50 wt %) resin component.

The resin component used in the curable flux composition of the presentinvention, optionally, further includes a material having at least threeoxirane groups per molecule. Material having at least three oxiranegroups per molecule is optionally included in the resin component toincrease the glass transition temperature of the cured resin product andto reduce the gellation time for the formulation.

Preferably, the carboxylic acid used in the flux composition of thepresent invention, is selected from C₈₋₂₀ aliphatic mono carboxylicacids; C₂₋₂₀ aliphatic dicarboxylic acids; C₆₋₂₀ aromatic carboxylicacids; and, mixtures thereof. More preferably, the carboxylic acid usedin the flux composition of the present invention is selected fromoctanoic acid; nonanioc acid; undecanoic acid; dodecanoic acid;tridecanoic acid; tetradecanoic acid; pentadecanoic acid; hexadecanoicacid; heptadecanoic acid; stearic acid; hydroxy stearic acid; oleicacid; linoleic acid; α-linolenic acid; icosanoic acid; oxalic acid;malonic acid; succinic acid; malic acid; glutaric acid; adipic acid;pimelic acid; suberic acid; benzoic acid; phthalic acid; isophthalicacid; terephthalic acid; hemimellitic acid; trimellitic acid; trimesicacid; mellophanic acid; prehnitic acid; pyromellitic acid; melliticacid; toluic acid; xylic acid; hemellitic acid; mesitylene acid;prehnitic acid; cinnamic acid; salicylic acid; benzoic acid (e.g.,benzoic acid; 2,3-dihydroxybenzoic acid; 2,4-dihydroxybenzoic acid;2,5-dihydroxybenzoic acid (gentisic acid); 2,6-dihydroxybenzoic acid;3,5-dihydroxybenzoic acid; 3,4,5-trihydroxybenzoic acid (gallic acid));naphthoic acid (e.g., naphthoic acid; 1,4-dihydroxy-2-naphthoic acid;3,5-dihydroxy-2-naphthoic acid; 3,7-dihydroxy-2-naphthoic acid);phenolphthalin; diphenolic acid and mixtures thereof. Still morepreferably, the carboxylic acid used in the flux composition of thepresent invention is selected from naphthoic acid (e.g., naphthoic acid;1,4-dihydroxy-2-naphthoic acid; 3,5-dihydroxy-2-naphthoic acid;3,7-dihydroxy-2-naphthoic acid), stearic acid; hydroxy stearic acid;oleic acid; linoleic acid; α-linolenic acid; and icosanoic acid; and,mixtures thereof. Yet still more preferably, the carboxylic acid used inthe flux composition of the present invention is selected from naphthoicacid (e.g., naphthoic acid; 1,4-dihydroxy-2-naphthoic acid;3,5-dihydroxy-2-naphthoic acid; 3,7-dihydroxy-2-naphthoic acid), stearicacid; hydroxy stearic acid; oleic acid; and, mixtures thereof. Mostpreferably, the carboxylic acid used in the flux composition of thepresent invention is selected from naphthoic acid, a C₁₈ carboxylic acidand mixtures thereof; wherein the naphthoic acid is selected from1,4-dihydroxy-2-naphthoic acid; 3,5-dihydroxy-2-naphthoic acid; and theC₁₈ carboxylic acid is selected from stearic acid, hydroxy stearic acid,and oleic acid.

The fluxing agent used in the curable flux composition of the presentinvention is according to formula I, wherein R¹, R², R³ and R⁴ areindependently selected from a hydrogen, a substituted C₁₋₈₀ alkyl group,an unsubstituted C₁₋₈₀ alkyl group, a substituted C₇₋₈₀ arylalkyl groupand an unsubstituted C₇₋₈₀ arylalkyl group (preferably wherein R¹, R²,R³ and R⁴ are independently selected from a hydrogen, a substitutedC₁₋₂₀ alkyl group, an unsubstituted C₁₋₂₀ alkyl group, a substitutedC₇₋₃₀ arylalkyl group and an unsubstituted C₇₋₃₀ arylalkyl group);wherein zero to three of R¹, R², R³ and R⁴ is(are) a hydrogen.Preferably, one to three of R¹, R², R³ and R⁴ is(are) a hydrogen. Morepreferably, two to three of R¹, R², R³ and R⁴ are a hydrogen. Still morepreferably, two of R¹, R², R³ and R⁴ are a hydrogen. Most preferably,one of R¹ and R² is a hydrogen and one of R³ and R⁴ is a hydrogen. TheR¹, R², R³ and R⁴ groups of the fluxing agent represented by formula Iare preferably selected: to provide the fluxing agent with desirablerheological properties for a given application; to facilitate theformation of the fluxing complex with the carboxylic acid; optionally,to compatibilize the fluxing agent with a given solvent package fordelivery to the surface(s) to be soldered; and, optionally, tocompatibilize the fluxing agent with a given encapsulating composition(e.g., an epoxy resin) to be used post soldering to form an encapsulatedsolder joint (e.g., for use in conventional flip chip under fillapplications). Also, the R¹, R², R³ and R⁴ groups of the fluxing agentaccording to formula I are preferably selected to provide the fluxingagent with a boiling point temperature of ≧250° C. (more preferably≧300° C.; most preferably ≧325° C.), determined by differential scanningcalorimetry using a ramp of 10° C./min starting at 25° C.

More preferably, the fluxing agent used in the curable flux compositionof the present invention is according to formula I; wherein R¹, R², R³and R⁴ are independently selected from a hydrogen, a substituted C₁₋₈₀alkyl group, an unsubstituted C₁₋₈₀ alkyl group, a substituted C₇₋₈₀arylalkyl group and an unsubstituted C₇₋₈₀ arylalkyl group; wherein thesubstitutions in the substituted C₁₋₈₀ alkyl group and the substitutedC₇₋₈₀ arylalkyl group are selected from at least one of an —OH group, an—OR⁵ group, a —COR⁵— group, a —COR⁵ group, a —C(O)R⁵ group, a —CHOgroup, a —COOR⁵ group, an —OC(O)OR⁵ group, a —S(O)(O)R⁵ group, a —S(O)R⁵group, a —S(O)(O)NR⁵ ₂ group, an —OC(O)NR⁶ ₂ group, a —C(O)NR⁶ ₂ group,a —CN group, a —N(R⁶)— group and a —NO₂ group (preferably at least oneof an —OH group, an —OR⁵ group, a —COR⁵— group, a —COR⁵ group, a —C(O)R⁵group, a —CHO group, a —COOR⁵ group, an —OC(O)OR⁵ group, a —S(O)(O)R⁵group, a —S(O)R⁵ group, a —S(O)(O)NR⁵ ₂ group, an —OC(O)NR⁶ ₂ group, a—C(O)NR⁶ ₂ group, a —CN group and a —NO₂ group); wherein R⁵ is selectedfrom a C₁₋₂₈ alkyl group, a C₃₋₂₈ cycloalkyl group, a C₆₋₁₅ aryl group,a C₇₋₂₈ arylalkyl group and a C₇₋₂₈ alkylaryl group; wherein R⁶ isselected from a hydrogen, a C₁₋₂₈ alkyl group, a C₃₋₂₈ cycloalkyl group,a C₆₋₁₅ aryl group, a C₇₋₂₈ arylalkyl group and a C₇₋₂₈ alkylaryl group;and, wherein zero to three of R¹, R², R³ and R⁴ is(are) a hydrogen. Thesubstituted C₁₋₈₀ alkyl group and the substituted C₇₋₈₀ arylalkyl groupcan contain combinations of substitutions. For example, the substitutedC₁₋₈₀ alkyl group and the substituted C₇₋₈₀ arylalkyl group can: containmore than one of the same type of substitution (e.g., two —OH groups);contain more than one type of substitution (e.g., an —OH group and a—COR⁵— group); contain more than one type of substitution with more thanone of the same type of substitution (e.g., two —OH groups and two—COR⁵— groups). Preferably, one to three of R¹, R², R³ and R⁴ is(are) ahydrogen. More preferably, two to three of R¹, R², R³ and R⁴ are ahydrogen. Still more preferably, two of R¹, R², R³ and R⁴ are ahydrogen. Most preferably, one of R¹ and R² is a hydrogen and one of R³and R⁴ is a hydrogen.

Still more preferably, the fluxing agent used in the curable fluxcomposition of the present invention is according to formula I; whereinR¹, R², R³ and R⁴ are independently selected from a hydrogen, asubstituted C₁₋₂₀ alkyl group, an unsubstituted C₁₋₂₀ alkyl group, asubstituted C₇₋₃₀ arylalkyl group and an unsubstituted C₇₋₃₀ arylalkylgroup; wherein the substitutions in the substituted C₁₋₂₀ alkyl groupand the substituted C₇₋₃₀ arylalkyl group are selected from at least oneof an —OH group, an —OR⁷ group, a —COR⁷— group, a —COR⁷ group, a —C(O)R⁷group, a —CHO group, a —COOR⁷ group, an —OC(O)OR⁷ group, a —S(O)(O)R⁷group, a —S(O)R⁷ group, a —S(O)(O)NR⁷ ₂ group, an —OC(O)NR⁸ ₂ group, a—C(O)NR⁸ ₂ group, a —CN group, a —N(R⁸)— group and a —NO₂ group(preferably at least one of an —OH group, an —OR⁷ group, a —COR⁷— group,a —COR⁷ group, a —C(O)R⁷ group, a —CHO group, a —COOR⁷ group, an—OC(O)OR⁷ group, a —S(O)(O)R⁷ group, a —S(O)R⁷ group, a —S(O)(O)NR⁷ ₂group, an —OC(O)NR⁸ ₂ group, a —C(O)NR⁸ ₂ group, a —CN group and a —NO₂group); wherein R⁷ is selected from a C₁₋₁₉ alkyl group, a C₃₋₁₉cycloalkyl group, a C₆₋₁₅ aryl group, a C₇₋₁₉ arylalkyl group and aC₇₋₁₉ alkylaryl group; wherein R⁸ is selected from a hydrogen, a C₁₋₁₉alkyl group, a C₃₋₁₉ cycloalkyl group, a C₆₋₁₅ aryl group, a C₇₋₁₉arylalkyl group and a C₇₋₁₉ alkylaryl group; and, wherein zero to threeof R¹, R², R³ and R⁴ is(are) a hydrogen. The substituted C₁₋₂₀ alkylgroup and the substituted C₇₋₃₀ arylalkyl group can contain combinationsof substitutions. For example, the substituted C₁₋₂₀ alkyl group and thesubstituted C₇₋₃₀ arylalkyl group can: contain more than one of the sametype of substitution (e.g., two —OH groups); contain more than one typeof substitution (e.g., an —OH group and a —COR⁷— group); contain morethan one type of substitution with more than one of the same type ofsubstitution (e.g., two —OH groups and two —COR⁷— groups). Preferably,one to three of R¹, R², R³ and R⁴ is(are) a hydrogen. More preferably,two to three of R¹, R², R³ and R⁴ are a hydrogen. Still more preferably,two of R¹, R², R³ and R⁴ are a hydrogen. Most preferably, one of R¹ andR² is a hydrogen and one of R³ and R⁴ is a hydrogen.

Yet still more preferably, the fluxing agent used in the fluxingcomposition of the present invention is according to formula I; whereinR¹, R², R³ and R⁴ are independently selected from a hydrogen, a—CH₂CH(OH)R⁹ and a —CH₂CH(OH)CH₂—O—R⁹ group; wherein R⁹ is selected froma hydrogen, a C₁₋₂₈ alkyl group, a C₃₋₂₈ cycloalkyl group, a C₆₋₁₆ arylgroup, a C₇₋₂₈ arylalkyl group and a C₇₋₂₈ alkylaryl group (preferably,wherein R⁹ is selected from a C₅₋₁₀ alkyl group, a C₆₋₁₆ aryl group anda C₇₋₁₅ alkylaryl group; more preferably wherein R⁹ is selected from aC₈ alkyl group, a C₇ alkylaryl group, a naphthyl group, a biphenyl groupand a substituted C₁₂₋₁₆ biphenyl group; most preferably, wherein R⁹ isselected from a C₈ alkyl group, a C₇ alkylaryl group and a naphthylgroup); and wherein zero to three of R¹, R², R³ and R⁴ is(are) ahydrogen. Preferably, one to three of R¹, R², R³ and R⁴ is(are) ahydrogen. More preferably, two to three of R¹, R², R³ and R⁴ are ahydrogen. Still more preferably, two of R¹, R², R³ and R⁴ are ahydrogen. Most preferably, one of R¹ and R² is a hydrogen and one of R³and R⁴ is a hydrogen.

Yet still more preferably, the fluxing agent used in the curable fluxcomposition of the present invention is according to formula I; whereinR¹ and R³ are a hydrogen; wherein R² and R⁴ are a —CH₂CH(OH)CH₂—O—R⁹group; wherein R⁹ is selected from a C₈ alkyl group, a C₇ alkylarylgroup and a C₁₀ naphthyl group.

Preferably, the curable flux composition of the present inventioncomprises, as initial components: a carboxylic acid and a fluxing agentrepresented by formula I at a fluxing agent amine nitrogen to carboxylicacid content (—COOH) equivalent ratio of 1:1 to 20:1 (more preferably,1:1 to 10:1; most preferably, 1:1 to 4:1). Preferably, when combined,the carboxylic acid and the fluxing agent represented by formula I, forma fluxing complex. Preferably, the fluxing complex formed is anacid-base complex. Preferably, the fluxing complex exhibits a percentweight loss of ≦25 wt % (more preferably ≦20 wt %; most preferably ≦15wt %) upon heating to 230° C. determined by thermogravimetric analysis(TGA) using a temperature ramp of 10° C./min starting at 25° C.

Optionally, the fluxing agent used in the fluxing composition of thepresent invention is a dimer having a formula selected from formula IIa,IIb and IIC; wherein the dimer contains a first mer according to formulaI and a second mer according to formula I; wherein one of R¹, R², R³ andR⁴ of the first mer and one of R¹, R², R³ and R⁴ of the second mercoincide joining the first mer and the second mer.

An example of a dimer according to formula (IIa) is1-(2-ethylhexyloxy)-3-(4-(2-(3-(4-(2-(4-(3-(4-(2-(3-(heptan-3-yloxy)-2-hydroxypropylamino)propan-2-yl)-1-methylcyclohexylamino)-2-hydroxypropoxy)phenyl)propan-2-yl)phenoxy)-2-hydroxypropylamino)propan-2-yl)-1-methylcyclohexylamino)propan-2-ol,as follows:

The curable flux composition of the present invention, optionally,further comprises a curing agent. Conventional curing agents can be usedtogether with the resin component and the fluxing agent, provided thatthe curing agents can cure the resin component. Conventional curingagents include, for example, polyfunctional phenols, polyfunctionalalcohols, amines, imidazole compounds, acid anhydrides, organicphosphorus compounds and their halides. Preferably, any curing agentused should be a latent curing agent (i.e., curing agent that does notoperate to initiate the gelling of the resin component at temperaturesof ≦225° C.) allowing for solder reflow without interference andfacilitating storage stability for curable flux compositions in one packsystems.

The curable flux composition of the present invention, optionally,further comprises a solvent. Solvent is optionally included in thecurable flux composition of the present invention to facilitate deliveryof the resin component and the fluxing agent to the surface, orsurfaces, to be soldered. Preferably, the curable flux compositioncontains 1 to 70 wt % solvent (more preferably 1 to 35 wt % solvent;most preferably 1 to 20 wt % solvent). Solvent used in the curable fluxcomposition of the present invention is preferably an organic solventselected from hydrocarbons (e.g., dodecane, tetradecane); aromatichydrocarbons (e.g., benzene, toluene, xylene, trimethylbenzene, butylbenzoate, dodecylbenzene); ketones (e.g., methyl ethyl ketone, methylisobutyl ketone, cyclohexanone); ethers (e.g., tetrahydrofuran,1,4-dioxaneandtetrahydrofuran, 1,3-dioxalane, diprolylene glycoldimethyl ether); alcohols (e.g., 2-methoxy-ethanol, 2-butoxyethanol,methanol, ethanol, isopropanol, α-terpineol, benzyl alcohol,2-hexyldecanol,); esters (e.g., ethyl acetate, ethyl lactate, butylacetate, diethyl adipate, diethyl phthalate, diethylene glycol monobutylacetate, propylene glycol monomethyl ether acetate, ethyl lactate,methyl 2-hydroxyisobutyrate, propylene glycol monomethyl ether acetate);and, amides (e.g., N-methylpyrrolidone, N,N-dimethylformamide andN,N-dimethylacetamide); glycol derivatives (e.g., cellosolve, butylcellosolve); glycols (e.g., ethylene glycol; diethylene glycol;dipropylene glycol; triethylene glycol; hexylene glycol;1,5-pentanediol); glycol ethers (e.g., propylene glycol monomethylether, methyl carbitol, butyl carbitol); and petroleum solvents (e.g.,petroleum ether, naptha). More preferably, the solvent used in thecurable flux composition of the present invention is an organic solventselected from methyl ethyl ketone; 2-propanol; propylene glycolmonomethyl ether; propylene glycol monomethyl ether acetate; ethyllactate and methyl 2-hydroxy isobutyrate. Most preferably, the solventused in the curable flux composition of the present invention ispropylene glycol monomethyl ether.

The curable flux composition of the present invention, optionally,further comprises a thickening agent. Preferably, the curable fluxcomposition contains 0 to 30 wt % thickening agent. Thickening agentused in the curable flux composition of the present invention can beselected from non-curing resin materials (i.e., <2 reactive functionalgroups per molecule), such as, for example, a non-curing novolac resin.The thickening agent used in the curable flux composition, preferablyexhibits a viscosity greater than that exhibited by the resin component(in its uncured form). When present, the thickening agent can be used inan amount of 0.1 to 35 wt %, based on the total weight of the curableflux composition.

The curable flux composition of the present invention, optionally,further comprises a thixotropic agent. Preferably, the curable fluxcomposition contains 1 to 30 wt % thixotropic agent. Thixotropic agentused in the curable flux composition of the present invention can beselected from fatty acid amides (e.g., stearamide, hydroxystearic acidbisamide); fatty acid esters (e.g., castor wax, beeswax, carnauba wax);organic thixotropic agents (e.g., polyethylene glycol, polyethyleneoxide, methyl cellulose, ethyl cellulose, hydroxyethyl cellulose,hydroxypropyl cellulose, diglycerine monooleate, deglycerine laurate,decaglycerine oleate, diglycerine monolaurate, sorbitan laurate);inorganic thixotropic agents (e.g., silica powders, kaolin powders).Preferably, the thixotropic agent used is selected from a polyethyleneglycol and a fatty acid amide.

The curable flux composition of the present invention, optionally,further comprise an inorganic filler. Inorganic fillers can be selectedfrom alumina, aluminum hydroxide, aluminosilicate, cordierite, lithiumaluminum silicate, magnesium aluminate, magnesium hydroxide, clay, talc,antimony trioxide, antimony pentoxide, zinc oxide, colloidal silica,fused silica, glass powder, quartz powder and glass microspheres. Whenpresent, the curable flux composition of the present inventionpreferably contains 0 to 70 wt % (more preferably 0 to 35 wt %, stillmore preferably 0 to 20 wt %; most preferably 0.1 to 20 wt %) inorganicfiller.

The curable flux composition of the present invention, optionally,further comprises an antioxidant. When present, the curable fluxcomposition of the present invention preferably contains 0.01 to 30 wt %(more preferably 0.01 to 20 wt %) antioxidant.

The curable flux composition of the present invention optionally furthercomprises a reactive diluent. Preferably, the optional reactive diluentshould exhibit a viscosity that is lower than the viscosity exhibited bythe resin component (in its uncured form). Reactive diluent canpreferably be selected from monofunctional epoxies (e.g., C₆₋₂₈ alkylglycidyl ethers; C₆₋₂₈ fatty acid glycidyl esters; C₆₋₂₈ alkylphenolglycidyl ethers) and certain multifunctional epoxies (e.g.,trimethylolpropane triglycidyl ether; diglycidyl aniline). When present,the reactive diluent can be used in an amount of <50 wt % (based on theweight of the resin component).

The curable flux composition of the present invention, optionally,further comprises a conventional air release agent. Air release agentsare believed to enhance the wetting of the surfaces to be solderedduring solder reflow. When present, the air release agent can be used inan amount of <1 wt %, based on the total weight of the curable fluxcomposition.

The curable flux composition of the present invention, optionally,further comprises a conventional defoaming agent. Defoaming agents arebelieved to enhance the wetting of the surfaces to be soldered duringsolder reflow and to reduce gas inclusion defects upon curing of thecurable flux composition. When present, the defoaming agent can be usedin an amount of <1 wt % of the curable flux composition.

The curable flux composition of the present invention, optionally,further comprises a conventional adhesion promoter. Conventionaladhesion promoters include silanes, such as, for example,glycidoxypropyl trimethoxysilane; γ-amino propyl trithosysilane;trimethoxysilylpropylated isocyanurate;β-(3,4-epoxycyclohexyl)ethyltriethoxysilane; glycidopropyldiethoxymethylsilane; β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane;γ-glycidoxypropyltriethoxysilane; γ-mercaptopropyltrimethoxysilane;N-beta-(aminoethyl)-gama-aminopropyltrimethoxysilane;bis(trimethoxysilylpropyl)amine; and, γ-ureidopropyltriethoxysilane.When present, the adhesion promoter can be used in an amount of <2 wt %of the curable flux composition.

The curable flux composition of the present invention, optionally,further comprises a conventional flame retardant. Conventional flameretardants include bromo compounds (e.g., decabromodiphenyl ether,tetrabromobisphenol A, tetrabromophthalic anhydride, tribromophenol);phosphorus compounds (e.g., triphenyl phosphate, tricresyl phosphate,trixylyl phosphate, cresyl diphenyl phosphate); metal hydroxides (e.g.,magnesium hydroxide, aluminum hydroxide); red phosphorus and itsmodified products; antimony compounds (e.g., antimony trioxide, antimonypentoxide); and, triazine compounds (e.g., melamine, cyanuric acid,melamine cyanurate). When present, the flame retardant can be used in anamount of 0.01 to 35 wt % (preferably 0.01 to 10 wt %) of the curableflux composition.

The curable flux composition of the present invention, optionally,further comprises additional additives selected from matting agents,coloring agents, dispersion stabilizers, chelating agents, thermoplasticparticles, UV impermeable agents, leveling agents and reducing agents.

The curable flux composition of the present invention can be provided asa one pack system containing all of the ingredients. Alternatively, thecurable flux composition can be provided as a two pack system; whereinthe resin component is provided in a first part and the fluxing agentand, optional, curing agent are provided in a second part; and, whereinthe first part and the second part are combined prior to use.

The curable flux composition of the present invention can be used in,for example, the production of electronic components, electronic modulesand printed circuit boards. The curable flux composition can be appliedto the surface(s) to be soldered by any conventional technique includingliquid spray techniques, liquid foaming techniques, pick and diptechniques, wave techniques, or any other conventional technique capableof dispensing a liquid or semisolid onto a silicon die or substrate.

The curable flux composition of the present invention optionally furthercomprises a solder powder; wherein the curable flux composition is asolder paste. Preferably, the solder powder is an alloy selected fromSn/Pb, Sn/Ag, Sn/Ag/Cu, Sn/Cu, Sn/Zn, Sn/Zn/Bi, Sn/Zn/Bi/In, Sn/Bi andSn/In (preferably wherein the solder powder is an alloy selected from 63wt % Sn/37 wt % Pb; 96.5 wt % Sn/3.5 wt % Ag; 96 wt % Sn/3.5 wt % Ag/0.5wt % Cu; 96.4 wt % Sn/2.9 wt % Ag/0.5 wt % Cu; 96.5 wt % Sn/3 wt %Ag/0.5 wt % Cu; 42 wt % Sn/58 wt % Bi; 99.3 wt % Sn/0.7 wt % Cu; 91 wt %Sn/9 wt % Zn and 89 wt % Sn/8 wt % Zn/3 wt % Bi).

The solder paste preferably comprises: 1 to 50 wt % (more preferably 5to 30 wt %, most preferably 5 to 15 wt %) of a fluxing complex formed bythe combination of a carboxylic acid with a fluxing agent represented byformula I; and, 50 to 99 wt % of a solder powder. The solder paste canbe compounded by conventional techniques, for example, by kneading andmixing the solder powder with the fluxing complex using conventionalequipment for such operations.

The solder paste can be used in, for example, the production ofelectronic components, electronic modules and printed circuit boards.The solder paste can be applied to the surface(s) to be soldered by anyconventional technique including printing the solder paste through aconventional solder mask using a solder printer or screen printer.

The fluxing agent represented by formula I used in the curable fluxcomposition of the present invention can be prepared using conventionalsynthesis techniques well known to those of ordinary skill in the art.

The fluxing complex used in the curable flux composition of the presentinvention can be prepared by, for example: (a) combining a fluxing agentaccording to formula I with a carboxylic acid (see, e.g., Example 8); or(b) adding a carboxylic acid at some point during the preparation of afluxing agent according to formula I. For example, under (b), acarboxylic acid can be mixed with menthane diamine (i) before reactingthe menthane diamine to form a fluxing agent according to formula I(see, e.g., Example 9); or, (ii) at some point during the reaction ofmenthane diamine to form a fluxing agent according to formula I (see,e.g., Example 10). Optionally, a fluxing agent according to formula Iand a carboxylic acid can be combined in a solvent (e.g., 1,3-dioxolane)to facilitate the formation of the fluxing complex. The solvent can thenbe evaporated off leaving behind the fluxing complex.

The method of forming a plurality of encapsulated electricalinterconnects of the present invention can optionally be part of a flipchip soldering process, wherein a semiconductor chip is mounted onto aprinted circuit board, wherein the semiconductor chip comprises aplurality of first electrical contacts and wherein the printed circuitboard comprises a plurality of corresponding second electrical contacts.In such a flip chip process, the curable flux composition of presentinvention is applied to either one, or both, of the plurality of firstelectrical contacts and the plurality of corresponding second electricalcontacts to facilitate solder bonding of the plurality of firstelectrical contacts to the plurality of corresponding second electricalcontacts to form electrical inter connects. Preferably, the flip chipsolder process further comprises a curing step wherein the resincomponent is cured, encapsulating the electrical inter connects betweenthe plurality of first electrical contacts and the plurality of secondelectrical contacts.

During the method of forming a plurality of encapsulated electricalinter connects of the present invention, the curable flux composition ofthe present invention is preferably applied onto a printed circuitboard, with or without a smoothing application, over the plurality offirst electrical contacts. The solder is preferably applied to theplurality of corresponding second electrical contacts on thesemiconductor chip in the form of solder balls. The semiconductor chipwith solder attached is then disposed over the curable flux compositiontreated printed circuit board. The semiconductor chip is then alignedwith the printed circuit board and the solder is heated above its reflowtemperature. During reflow, the curable flux composition acts as a fluxand promotes adhesive of the solder to the first plurality of electricalcontacts on the printed circuit board forming a plurality of electricalinter connects between the plurality of first electrical contacts andthe plurality of corresponding second electrical contacts. The resincomponent is then cured to encapsulate the plurality of electrical interconnects.

During reflow, the solder melts and flows to form the plurality ofelectrical interconnects. The curable flux composition preferably shouldnot gel until after the solder has flowed and forms the plurality ofelectrical interconnects, otherwise the printed circuit board and thesemiconductor chip may not align correctly. It is preferably that theresin component in the curable flux composition completely curefollowing solder reflow to form an encapsulated electrical joint.

Some embodiments of the present invention will now be described indetail in the following Examples.

Examples 1-5 Synthesis of Fluxing Agent

The fluxing agents identified in TABLE 1 were prepared from thematerials identified in TABLE 1 using the following procedure.Specifically, into a reaction vessel with a stir bar, (0.1 mol) menthanediamine (available from The Dow Chemical Company as Primene™ MD) wasadded. The reaction vessel was then placed on a hotplate with magneticstirring capability. The reaction vessel was then inerted with nitrogenand (0.2 mol) Reagent A identified in TABLE 1 was then added to thereaction vessel at ambient temperature, with stirring. The set pointtemperature on the hot plate was then raised to 75° C. and the contentsof the reaction vessel were allowed to continue stirring for two (2)hours. The set point temperature of the hot plate was then raised to140° C. and the contents of the reaction vessel were allowed to continuestirring for two (2) more hours. The set point temperature of the hotplate was then reduced to 80° C. and a vacuum was pulled on the reactionvessel, reducing the pressure in the vessel to 30 mm Hg. The contents ofthe reaction vessel were allowed to continue stirring under theseconditions for another two (2) hours to provide the product fluxingagent as identified in TABLE 1. The boiling point temperature of theproduct fluxing agents for Examples 1-3 were then measured bydifferential scanning calorimetry (DSC) using a ramp of 10° C./min. from25° C. to 500° C. The measured boiling point temperature (BPT) for theproduct fluxing agent for Examples 1-3 is provided in TABLE 1. Also, thepercent weight loss from the product fluxing agent for Examples 1-3 uponheating to 250° C. was measured by thermogravimetric analysis (TGA)using a temperature ramp of 10° C./min starting at 25° C. The measuredweight loss (WL) for the product fluxing agent for Examples 1-3 isprovided in TABLE 1.

TABLE 1 Ex. Reagent A Fluxing Agent 1 2-ethylhexyl glycidyl ether

BPT by DSC ≧ 325° C. WL by TGA = 9 wt % (@ 250° C.) 2 cresyl glycidylether

BPT by DSC ≧ 325° C. WL by TGA = 8 wt % (@ 250° C.) 3 1-naphthylglycidyl ether

BPT by DSC ≧ 350° C. WL by TGA = 8 wt % (@ 250° C.) 4 alkyl C₁₂₋₁₄glycidyl ether

5 p-t-butylphenyl glycidyl ether

Example 6 Synthesis of a Dimeric Fluxing Agent

A dimeric fluxing agent was prepared using the following procedure. Intoa reaction vessel with a stir bar, (34 g) of menthane diamine (availablefrom The Dow Chemical Company as Primene™) and (37.4 g) of a liquidepoxy resin reaction product of epichlorohydrin and bisphenol A(available from The Dow Chemical Company as D.E.R.™ 331™) were added.The reaction vessel was then placed on a hotplate with magnetic stirringcapability. The contents of the reaction vessel were left to stir atroom temperature overnight, turning into a solid. Then (37.2 g) of2-ethylhexyl glycidyl ether (available from Momentive PerformanceMaterials) was charged to the reaction vessel. The reaction vessel wasthen heated up to 150° C. with a one hour ramp from room temperature andthen held at 150° C. for three hours. The contents of the reactionvessel were then allowed to cool to room temperature to provide adimeric fluxing agent product.

Example 7 Synthesis of a Dimeric Fluxing Agent

Another dimeric fluxing agent was prepared using the followingprocedure. Into a reaction vessel with a stir bar, (34 g) of menthanediamine (available from The Dow Chemical Company as Primene™) and (37.4g) of a liquid epoxy resin reaction product of epichlorohydrin andbisphenol A (available from The Dow Chemical Company as D.E.R.™ 331™)were added. The reaction vessel was then placed on a hotplate withmagnetic stirring capability. The contents of the reaction vessel wereleft to stir at room temperature overnight, turning into a solid. Then(40.0 g) of 1-naphthyl glycidyl ether (available from MomentivePerformance Materials) was charged to the reaction vessel. The reactionvessel was then heated up to 75° C. and held at that temperature for onehour. The reaction vessel was then further heated to 150° C. with a onehour ramp from 75° C. and then held at 150° C. for three hours. Thecontents of the reaction vessel were then allowed to cool to roomtemperature to provide a dimeric fluxing agent product.

Example 8 Preparation of Fluxing Complex

Fluxing agent (18.07 g; 33.3 mmoles) prepared according to the procedureset forth in Example 1 was hand mixed with 1,4-dihydroxy-2-naphthoicacid (1.69 g; 8.3 mmoles) under ambient conditions using a spatula toform a fluxing complex having a fluxing agent amine nitrogen tocarboxylic acid content (—COOH) equivalent ratio of 2:1.

The percent weight loss from the fluxing complex upon heating to 230° C.was measured by thermogravimetric analysis (TGA) using a temperatureramp of 10° C./min starting at 25° C. The measured weight loss (WL) forthe fluxing complex was 14.2 wt %.

Example 9 Preparation of Fluxing Complex

Into a reaction vessel with a stir bar, (0.33 moles) menthane diamine(available from The Dow Chemical Company as Primene™ MD) was added. Thereaction vessel was then placed on a hotplate with magnetic stirringcapability. The reaction vessel was then inerted with nitrogen and (0.66moles) of 2-ethylhexyl glycidyl ether (from Hexion Specialty Chemicals)was then added to the reaction vessel at ambient temperature, withstirring. The set point temperature on the hot plate was then raised to75° C. and the contents of the reaction vessel were allowed to continuestirring for two (2) hours. The set point temperature of the hot platewas then raised to 140° C. and the contents of the reaction vessel wereallowed to continue stirring for two (2) more hours. The contents of thereaction vessel were then cooled down to 75° C. and1,4-dihydroxy-2-naphthoic acid (16.9 g; 0.083 moles) was then slowlyadded to the reaction vessel over 30 minutes. The set point temperatureof the hot plate was then set 80° C. and a vacuum was pulled on thereaction vessel, reducing the pressure in the vessel to 30 mm Hg. Thecontents of the reaction vessel were allowed to continue stirring underthese conditions for another two (2) hours to form a fluxing complex.

The percent weight loss from the fluxing complex upon heating to 230° C.was measured by thermogravimetric analysis (TGA) using a temperatureramp of 10° C./min starting at 25° C. The measured weight loss (WL) forthe fluxing complex was 14.7 wt %.

Example 10 Preparation of Fluxing Complex

Into a reaction vessel with a stir bar, (0.33 moles) menthane diamine(available from The Dow Chemical Company as Primene™ MD) was added. Thereaction vessel was then placed on a hotplate with magnetic stirringcapability. The reaction vessel was then inerted with nitrogen and(0.083 moles) of 1,4-dihydroxy-2-naphthoic acid was slowly added over 30minutes at ambient temperature, with stirring. (0.66 moles) of2-ethylhexyl glycidyl ether (from Hexion Specialty Chemicals) was thenadded to the reaction vessel at ambient temperature, with stirring. Theset point temperature on the hot plate was then raised to 75° C. and thecontents of the reaction vessel were allowed to continue stirring fortwo (2) hours. The set point temperature of the hot plate was thenraised to 140° C. and the contents of the reaction vessel were allowedto continue stirring for two (2) more hours. The set point temperatureof the hot plate was then set 80° C. and a vacuum was pulled on thereaction vessel, reducing the pressure in the vessel to 30 mm Hg. Thecontents of the reaction vessel were allowed to continue stirring underthese conditions for another two (2) hours to provide a fluxing complex.

The percent weight loss from the fluxing complex upon heating to 230° C.was measured by thermogravimetric analysis (TGA) using a temperatureramp of 10° C./min starting at 25° C. The measured weight loss (WL) forthe fluxing complex was 13.8 wt %.

Examples 11 Preparation of Curable Flux Composition

The fluxing complex prepared according to each one of Example 10 wascombined with a liquid epoxy resin reaction product of epichlorohydrinand bisphenol A (available from The Dow Chemical Company as D.E.R.™331™) at a 1:1 weight ratio to form a curable flux composition.

Example 12 Evaluation of Fluxing Capability

The fluxing capability of the curable flux composition preparedaccording to Example 11 was evaluated using the following procedure. Acopper coupon was used as an electrical contact to be soldered. A smalldrop of the curable flux composition prepared according to Example 11was dispensed onto the surface to be soldered of the copper coupon. Four0.381 mm diameter balls of a lead-free solder (95.5 wt % Sn/4.0 wt %Ag/0.5 wt % Cu) were placed into the drop of fluxing complex on thecopper coupon. The melting range of the lead-free solder used was 217 to221° C. The copper coupon was then placed on a hotplate that waspreheated to 145° C. and held there for two (2) minutes. The coppercoupons were then placed on another hotplate preheated to 260° C. andheld there until the solder reached reflow conditions. The copper couponwas then removed from the heat and evaluated by (a) the extent of fusionand coalescence of the originally placed four solder balls, (b) the sizeof the resulting coalesced solder to assess the flow and spread and (c)the bonding of the solder to the surface of the copper coupon. Thefluxing capability of the curable flux composition was determined to bea 4 on a scale of 0 to 4, wherein:

-   -   0=no fusion between solder drops and no solder bonding to copper        coupon;    -   1,2=partial to complete fusion between solder drops, but no        solder bonding to copper coupon;    -   3=complete fusion between solder drops, but minimal solder        spread and flow;    -   4=complete fusion between solder drops, good solder spread and        flow over surface of copper coupon and solder bonding to the        copper coupon.

We claim:
 1. A curable flux composition, comprising, as initial components: a resin component having at least two oxirane groups per molecule; a carboxylic acid; a fluxing agent represented by formula I:

wherein R¹, R², R³ and R⁴ are independently selected from a hydrogen, a substituted C₁₋₈₀ alkyl group, an unsubstituted C₁₋₈₀ alkyl group, a substituted C₇₋₈₀ arylalkyl group and an unsubstituted C₇₋₈₀ arylalkyl group; and wherein zero to three of R¹, R², R³ and R⁴ is(are) a hydrogen; and, optionally, a curing agent.
 2. The curable flux composition of claim 1, wherein the carboxylic acid is selected from the group consisting of a C₈₋₂₀ aliphatic mono carboxylic acids; C₂₋₂₀ aliphatic dicarboxylic acids; C₆₋₂₀ aromatic carboxylic acids; and, mixtures thereof.
 3. The curable flux composition of claim 2, wherein the carboxylic acid is selected from the group consisting of octanoic acid; nonanioc acid; undecanoic acid; dodecanoic acid; tridecanoic acid; tetradecanoic acid; pentadecanoic acid; hexadecanoic acid; heptadecanoic acid; stearic acid; hydroxy stearic acid; oleic acid; linoleic acid; α-linolenic acid; icosanoic acid; oxalic acid; malonic acid; succinic acid; malic acid; glutaric acid; adipic acid; pimelic acid; suberic acid; benzoic acid; phthalic acid; isophthalic acid; terephthalic acid; hemimellitic acid; trimellitic acid; trimesic acid; mellophanic acid; prehnitic acid; pyromellitic acid; mellitic acid; toluic acid; xylic acid; hemellitic acid; mesitylene acid; prehnitic acid; cinnamic acid; salicylic acid; benzoic acid; naphthoic acid; phenolphthalin; diphenolic acid and mixtures thereof.
 4. The curable flux composition of claim 1, wherein the flux composition exhibits a fluxing agent amine nitrogen to carboxylic acid content (—COOH) equivalent ratio of 1:1 to 20:1.
 5. The curable flux composition of claim 1, wherein the substitutions in the substituted C₁₋₈₀ alkyl group and the substituted C₇₋₈₀ arylalkyl group are selected from at least one of an —OH group, an —OR⁵ group, a —COR⁵— group, a —C(O)R⁵ group, a —COR⁵ group, a —CHO, a —COOR⁵ group, an —OC(O)OR⁵ group, a —S(O)(O)R⁵ group, a —S(O)R⁵ group, a —S(O)(O)NR⁵ ₂ group, an —OC(O)NR⁶ ₂ group, a —C(O)NR⁶ ₂ group, a —CN group, a —N(R⁶)— group and a —NO₂ group; wherein R⁵ is selected from a C₁₋₂₈ alkyl group, a C₃₋₂₈ cycloalkyl group, a C₆₋₁₅ aryl group, a C₇₋₂₈ arylalkyl group and a C₇₋₂₈ alkylaryl group; and, wherein R⁶ is selected from a hydrogen, a C₁₋₂₈ alkyl group, a C₃₋₂₈ cycloalkyl group, a C₆₋₁₅ aryl group, a C₇₋₂₈ arylalkyl group and a C₇₋₂₈ alkylaryl group.
 6. The curable flux composition of claim 1, wherein one to three of R¹, R², R³ and R⁴ is(are) a hydrogen.
 7. The curable flux composition of claim 1, wherein R¹, R², R³ and R⁴ are independently selected from a hydrogen, a —CH₂CH(OH)R⁹ and a —CH₂CH(OH)CH₂—O—R⁹ group; wherein R⁹ is selected from a hydrogen, a C₁₋₂₈ alkyl group, a C₃₋₂₈ cycloalkyl group, a C₆₋₁₅ aryl group, a C₇₋₂₈ arylalkyl group and a C₇₋₂₈ alkylaryl group.
 8. The curable flux composition of claim 4, wherein one of R¹ and R² is a hydrogen; and wherein one of R³ and R⁴ is a hydrogen.
 9. The curable flux composition of claim 1, further comprising: a solder powder.
 10. A process of forming an encapsulated metallurgical joint, comprising: providing a curable flux composition according to claim 1; providing a plurality of first electrical contacts; providing a plurality of corresponding second electrical contacts; providing a solder; applying the curable flux composition to at least one of the plurality of first electrical contacts and the plurality of corresponding second electrical contacts; placing the plurality of first electrical contacts in proximity to the plurality of corresponding second electrical contacts; heating the solder above its reflow temperature forming a molten solder and exposing the molten solder to the plurality of first electrical contacts and the plurality of corresponding second electrical contacts; displacing the curable flux composition from the plurality of first electrical contacts and the plurality of corresponding second electrical contacts with the molten solder and forming a plurality of electrical inter connects between the plurality of first electrical contacts and the plurality of corresponding second electrical contacts; and, curing the curable thermosetting resin composition, encapsulating the plurality of electrical inter connects. 